Reduction of the rate of fuel consumption is one of the biggest technical themes in automobiles. As part of this, there have been increasingly strong demands made for the reduction of the weight of the pneumatic tires.
The inner surface of a pneumatic tire is, however, provided with an inner liner layer comprising a low gas permeation rubber such as a butyl rubber or a halogenated butyl rubber so as to maintain a constant tire air pressure. A halogenated butyl rubber, however, has a large hysteresis loss, so when rippling occurs in the inner surface rubber of the carcass layer and the inner liner layer at the intervals between carcass cords after vulcanization of the tire, the inner liner rubber layer will deform along with the deformation of the carcass layer, and therefore, there will be the problem of an increase of the rolling resistance. Accordingly, in general, a rubber sheet called a tie rubber with a small hysteresis loss is interposed between the inner liner layer (halogenated butyl rubber) and inner surface rubber of the carcass layer to adhere the two. Therefore, in addition to the thickness of the inner liner layer of the halogenated butyl rubber, the thickness of the tie rubber is added and the thickness of the layers as a whole exceeds 1 mm (1000 .mu.m) which in the end becomes a reason increasing the weight of the product.
Techniques have been proposed for using various materials instead of a low gas permeation rubber such as a butyl rubber as the inner liner layer of the pneumatic tire. For example, Japanese Examined Patent Publication (Kokoku) No. 47-31761 discloses coating the inner surface of a vulcanized tire with a solution or dispersion of synthetic resins such as polyvinylidene chloride, saturated polyester resin, polyamide resin having an air permeation coefficient (cm.sup.3 (standard state)/cm.multidot.sec.multidot.mmHg) of not more than 10.times.10.sup.-13 at 30.degree. C. and not more than 50.times.10.sup.-13 at 70.degree. C., at a thickness of 0.1 mm or less.
The technique disclosed in Japanese Examined Patent Publication (Kokoku) No. 47-31761, however, describes to provide on the inner circumferential surface of the carcass or the inner circumferential surface of the inner liner of the vulcanized tire a covering layer of a synthetic resin having a specific air permeation coefficient and to keep the thickness of the synthetic resin covering layer to 0.1 mm or less, but the pneumatic tire described in this publication has problems in the adhesiveness of the rubber and synthetic resin. Further, there is the defect that the inner liner layer is inferior in moisture resistance (or water resistance).
Japanese Unexamined Patent Publication (Kokai) No. 5-330307 discloses to halogenate the inner surface of the tire (using the conventionally known chlorination solution, bromine solution, or iodine solution) and then form over that a polymer film (thickness of 10 to 200 .mu.m) of methoxymethylated nylon, copolymerized nylon, a blend of polyurethane and polyvinylidene chloride, or a blend of polyurethane and polyvinylidene fluoride.
Further, Japanese Unexamined Patent Publication (Kokai) No. 5-318618 discloses a pneumatic tire having a thin film of methoxymethylated nylon as an inner liner. According to this technique, the inner surface of the green tire is sprayed or coated with a solution or emulsion of methoxymethylated nylon and the tire then vulcanized or the inner surface of the tire after vulcanization is sprayed or coated with a solution or emulsion of methoxymethylated nylon to produce a pneumatic tire. In the arts disclosed in these publications as well, however, in addition to the defect of the poor water resistance of the thin films, there is the defect of a difficulty in maintaining uniformity of the film thickness.
Further, Japanese Unexamined Patent Publication (Kokai) No. 6-40207 has an example of use of a multilayer film having a low air permeation layer comprising a polyvinylidene chloride film or an ethylene-vinyl alcohol copolymer film and an adhesive layer composed of a polyolefin film, an aliphatic polyamide film, or a polyurethane film as the air permeation preventive layer of the tire. In this system, however, the low air permeation layer lacks flexibility and the film cannot track expansion or contraction of the material when the tire is being run on so splits.
Further, Japanese Unexamined Patent Publication (Kokai) No. 5-508435 proposes the use, as a tire inner liner composition, of a composition comprised of a halogen-containing copolymer of C.sub.4 to C.sub.7 isomonoolef in and p-alkylstyrene containing carbon black, a plasticizer oil, and a vulcanization agent for the tire inner liner, but the inner liner has an insufficient air permeation coefficient and is not suitable for reducing the weight of the tire further.
That is, the material for forming the gas barrier layer used for the inner liner of a pneumatic tire etc. is required to have flexibility and a gas barrier property, but no material has yet been presented which has both of these properties.
A thermoplastic elastomer composition which is composed of a thermoplastic resin component as a continuous phase and an elastomer component as a dispersed phase and in which at least part of the elastomer component is cross-linked (vulcanized) has the rubber elasticity performance derived from the elastomer component which has generally been cross-linked in the past, and, due to the thermoplastic resin component forming the continuous phase, can be thermoplastically molded at a high temperature where it melts and becomes fluid, it is known.
That is, a thermoplastic elastomer composition having this dispersed structure has the characteristic of enabling formation by processing techniques similar to those of plastics while maintaining the properties of a vulcanized rubber.
Therefore, the above elastomer composition has the following basic advantages compared with vulcanized rubber:
(1) a vulcanization process is not required. PA1 (2) recycling of the products and the scrap produced during the processing are possible. PA1 (3) lightening of the weight is possible. PA1 .phi..sub.m : volume fraction of thermoplastic resin component (A) PA1 a=-0.0518, b=0.90 PA1 .eta..sub.A : melt viscosity of thermoplastic elastomer composition component during melt mixing PA1 .eta..sub.B : melt viscosity of adhesive thermoplastic resin component during melt mixing; and PA1 the volume fraction based upon the total amount of (A) and (B) is controlled to 1 to 40%. PA1 (A) at least one thermoplastic resin component having an air permeation coefficient of not more than 25.times.10.sup.-12 cc.multidot.cm/cm.sup.2 .multidot.sec.multidot.cmHg and a Young's modulus of more than 500 MPa in an amount of at least 10% by weight, based upon the weight of the overall polymer component, PA1 (B) at least one elastomer component having an air permeation coefficient of more than 25.times.10.sup.-12 cc.multidot.cm/cm.sup.2 .multidot.sec.multidot.cmHg and a Young's modulus of not more than 500 MPa in an amount of at least 10% by weight, based upon with respect to the weight of the overall polymer component, the total weight (A)+(B) of the component (A) and the component (B) being at least 30% by weight, based upon the weight of the overall polymer component, the component (A) forming a continuous phase, and the component (B) forming a dispersed phase, and PA1 (C), in the thermoplastic resin of the component (A), an adhesive thermoplastic resin component having a volume fraction x viscosity ratio with the thermoplastic resin shown by the following relation: ##EQU4## where, .phi..sub.A : volume fraction of thermoplastic resin component (A) .phi..sub.C : volume fraction of adhesive thermoplastic resin component (C) PA1 .eta..sub.A : melt viscosity of thermoplastic elastomer component (A) during melt mixing PA1 .eta..sub.C : melt viscosity of adhesive thermoplastic resin component (C) during melt mixing, PA1 in 1 to 75% by weight, based on the total weight of the components (A), (B), and (C). PA1 (A) at least one thermoplastic resin having an air permeation coefficient of not more than 25.times.10.sup.-12 cc.multidot.cm/cm.sup.2 .multidot.sec.multidot.cmHg and a Young's modulus of more than 500 MPa in an amount of at least 10% by weight, based upon the weight of the overall polymer component, PA1 (B) at least one elastomer component having an air permeation coefficient of more than 25.times.10.sup.-12 cc.multidot.cm/cm.sup.2 .multidot.sec.multidot.cmHg and a Young's modulus of not more than 500 MPa in an amount of at least 10% by weight with respect to the weight of the overall polymer component, PA1 (C), in the thermoplastic resin of the component (A), an epoxide of a block copolymer comprising a vinyl aromatic compound and a conjugated diene compound and/or its partial hydrogenate in an amount of 3 to 50% by weight.
Among these, in particular, a thermoplastic elastomer composition where part or all of the elastomer component forming the dispersed phase is cross-linked (vulcanized) with the thermoplastic resin forming the continuous phase during the mixing, that is, is dynamically cross-linked (vulcanized), can in particular give a product superior in the mechanical physical properties of a rubber elastomer, resistance to compression set, and resistance to oil and can be used, instead of conventional rubber, for auto parts, building materials, medical equipments, general industrial materials, etc.
Relating to this thermoplastic elastomer composition, the present inventors previously proposed and filed an application (Japanese Patent Application No. 7-8394) for a polymer composition for a tire having an air permeation coefficient of not more than 25.times.10.sup.-12 cc.multidot.cm/cm.sup.2 .multidot.sec.multidot.cmHg and a Young's modulus of 1 to 500 MPa which is superior in the balance of the air permeation preventive property and the flexibility as a polymer composition for a tire, which can be used to reduce the weight of the tire, and which is comprised of a blend of a specific amount of a thermoplastic resin having an air permeation coefficient of not more than 25.times.10.sup.-12 cc.multidot.cm/cm.sup.2 .multidot.sec.multidot.cmHg and a Young's modulus of more than 500 MPa and a specific amount of an elastomer component having an air permeation coefficient of more than 25.times.10.sup.-12 cc.multidot.cm/cm.sup.2 .multidot.sec.multidot.cmHg and a Young's modulus of not more than 500 MPa.
This proposed composition, however, had a superior function as a polymer composition for a tire, but when used as the inner liner layer or other air permeation preventive layer of a pneumatic tire had the problem that the adhesiveness with the rubber layer was not sufficient with the thermoplastic elastomer composition alone.
Further, the present applicant engaged in research to make the air permeation preventive property in the above polymer composition for a tire even more effective and proposed and filed an application (Japanese Patent Application No. 7-55929) for a thermoplastic resin composition for a tire forming an integral air permeation preventive layer by extruding a blend of at least two types of incompatible thermoplastic resins in the process of which one thermoplastic resin component among the thermoplastic resin components is not finally dispersed but is dispersed oriented in a flat form due to the shear stress at the time of extrusion as it is incompatible.
When this thermoplastic resin composition, however, is used as the air permeation preventive layer of a pneumatic tire, since it is a thermoplastic resin composition, a sufficient air permeation preventive property can be obtained, but this was not sufficient to sufficiently control the flexibility and durability with respect to flexural fatigue and also there was the problem of an insufficient adhesiveness with the rubber layer with the thermoplastic resin composition alone.
That is, as explained above, there is known a thermoplastic elastomer composition, having a thermoplastic resin component as a continuous phase and an elastomer component as a dispersed phase, which has rubber elasticity, is flexible, can be processed thermoplastically, and enables control of the air barrier property or gas barrier property. Further, there is known a thermoplastic resin composition which enables a layer having a gas permeation preventive property to be formed integrally inside the layers. A thermoplastic elastomer composition which has rubbery elasticity and can be thermoplastically processed and which enables the formation of a layer having bondability and other necessary functions integrally at the outside of the layers is not yet known.